The present invention relates to the field of packaging semiconductor devices, and more particularly to packaging light emitting diodes.
Light emitting devices (LEDS) such as light emitting diodes are often packaged within leadframe packages. A leadframe package typically includes an LED connected to thin metal leads where the LED and most of the leads are completely encapsulated within a plastic body. A part of the plastic body defines a lens. A portion of the leads connected to the LED extends outside the plastic body. The metal leads of the leadframe package serve as the conduit to supply the LED with electrical power and, at the same time, may act to draw heat away from the LED. Heat is generated by the LED when power is applied to the LED to produce light. The portion of the leads that extend out from the package body connects to circuits external to the leadframe package.
Some of the heat generated by the LED is dissipated by the plastic package body; however, most of the heat is drawn away from the LED via the metal components of the package. The metal leads are typically very thin and have small cross sections. For this reason, capacity of the metal leads to remove heat from the LED is limited. This limits the amount of power that can be sent to the LED. This, in turn, limits the amount of light that can be generated by the LED.
To increase the capacity of an LED package to dissipate heat, in one LED package design, a heat sink slug is introduced into the package. The heat sink slug draws heat from the LED chip, thus increasing the heat dissipating capacity of the LED package. However, this design introduces free space in the optical cavity of the package that needs to be filled with a refractive-index-matching clear encapsulant within the package to extract light from the LED chip. Unfortunately, the volumetric expansion and contraction of the encapsulant typically exceeds that of the space that contains it. Hence, as the temperature rises, the encapsulant expands and overflows or is oozed out from the cavity through vent holes. Further, when it cools, the encapsulant contracts, creating a partial vacuum inside the cavity, and thus causing air or moisture to be sucked in. Sometimes, voids are formed inside the encapsulant or it delaminates from various components that it comes into contact with. This adversely affects the light output and reliability of the package. Furthermore, this design commonly includes a pair of flimsy leads which are typically soldered by a hot-iron. This ironing process is incompatible with the commonly used SMT (Surface Mount Technology) electronic board assembly processes.
In another LED package design, leads of the leadframe are made of different thicknesses in various shapes and configurations and extend beyond the immediate edge of the LED package body. The package body is molded with transparent thermoset plastic which usually serves as the encapsulation material for the package. These leads are typically thicker than the leads of the LEDS of the previous design. The thicker lead is utilized as a heat-spreader and the LED chip is mounted on it. This arrangement allows heat generated by the LED chip to dissipate through the thicker leads which are connected thermally to an external heat sink. Unfortunately, this design is inherently unreliable due to large difference in coefficient of thermal expansion (CTE) between the plastic body, the encapsulant and the leadframe materials. Thus, when subjected to temperature cycles of, say,−40 C to 120 C, most or all components of the LED package experience high thermal stresses, especially at contact points. This frequently results in cracking of the LED chips, delamination of plastic body from the leads, breaking of bond wires, or a combination of these problems. In addition, the extended leads increase the size of the LED package size and footprint. The increased size prevents mounting of the packages in a dense cluster on a PCB (printed circuit board) to generate bright light for certain applications, for example, for automobile lighting or for general illumination.
Another disadvantage of the current high power leadframe package designs is that the thick lead material cannot be stamped into a fine circuit for flip-chip or mounting of several LED chips that can be addressed independently for color control.
Consequently, there remains a need for an improved LED package that overcomes or alleviates one or more of the shortcomings of the prior art packages.